1. Field of the Invention
This invention relates to electrochemical electrodes, such as polarographic sensors, which utilize a gas-permeable membrane over the sensing surface. In particular, this device relates to apparatus for applying and affixing the membrane to the electrode.
2. Description of the Prior Art
There are many electrochemical electrodes, such as polarographic sensors, which utilized a gas-permeable membrane between the electrode and the sample. A typical configuration is the well-known Clark Oxygen Polarographic Sensor, which includes an anode and a cathode in the sensor head in contact with an electrolyte and separated from the sample by a membrane permeable to the gas to be measured but impermeable to the electrolyte. Suitable materials for such membranes are plastics such as polyethylene, polypropylene, polytetrafluoraethylene, polyester, silicone rubber and the like.
Many such sensors include a sensing surface of metal or metal imbedded in glass against which the membrane is stretched and attached by a clamping ring. A thin layer of electrolyte is trapped between the sensing surface and the membrane. It is critical to control the thicknesses of the membrane and of the electrolyte layer in order to make the measurements achieved with such electrodes reproducible. It is also necessary to ensure that the attachment of the membrane to the sensing surface by the clamping ring is sufficiently secure to prevent the leakage of any electrolyte to the surrounding medium. Any such leakage could make the attempted measurements useless. Leakage also causes the electrolyte and membrane to dry out too fast.
Controlling the thicknesses and tightness is complicated by the fact that the membranes must be replaced on some electrodes very frequently, as often as every few hours. This is often done in clinical or laboratory surroundings by technicians not specifically trained in this procedure. Reproducibility becomes therefore very difficult to achieve.
One conventional approach is to use a holder which positions the electrode while a membrane can be carefully applied by hand. The unstretched membrane is positioned on the sensor surface and pressed into place by an applicator top. The top is then removed and used, during a second step, to press a retainer ring onto the membrane to hold it in place. This technique has the disadvantage that undesired tensions and/or wrinkles can be produced in the membrane which alter the thickness of the membrane and therefore the electrolyte in an uncontrollable manner and consequently may result in leakage.
Another approach utilizes a cap-shaped membrane as shown in U.S. patent application Ser. No. 805,704, filed June 13, 1977. This cap-shaped membrane is preformed in order to reduce the affects of stretching and stresses on the thickness of the membrane. Achieving reproducibility with these methods, however, is both time consuming and difficult.